The problem of fluid loss to highly porous underground formations penetrated by a well has been long recognized. These highly porous zones are often called thief zones. In water or steam stimulation operations, for example, a serious problem is often encountered because a very small interval of the total production zone may be taking 80 percent or more of the total injected fluid. When this happens, the benefit of the injection project may be lost or greatly reduced.
An isolated high-permeability zone or fracture can be plugged at the well bore face by a shallow layer of applied cement, though such a permanent relatively irrevocable technique often is undesirable. More desirably, a communicating high-permeability zone is plugged to some considerable depth in order to prevent flood water from otherwise merely flowing around a narrow shallow plug and back into the high-permeability or swept zone. In-depth plugging of a relatively high-permeability zone converts the zone into a much lower permeability zone. Then, subsequently injected flood water or other fluid will tend to enter the formerly by-passed but now relatively more permeable hydrocarbon-bearing zones and thus mobilize increased amounts of hydrocarbons.
Various methods have been used in the past to achieve in-depth gelling, such as gelable systems triggered by a following aqueous acidic solution injection for subsequent pH adjustment. However, injecting an acidic solution following the gelable solution may result in such rapid gelation that sufficient in-depth plugging is not obtained in the most permeable strata where desired. In another method, water, a polymer and a cross-linking agent capable of gelling the polymer such as a sequestered polyvalent metal cation, are admixed, and, just before injection into an underground formation, an acid is added thereto to effect gelation. But, when the acid is pre-mixed with the gelable composition, the gelation can be too fast, making it necessary to shear the gelled polymer in order to obtain adequate injection, which reduces effectiveness of the gel.
Other methods have used various compounds and procedures to control gelling of alkali metal silicates in underground formations. These methods often have problems with premature gelling so that deep penetration in the formation is not achieved.
According to this invention substantial reduction in permeability of a highly permeable zone in an underground formation is achieved by introducing into the formation through a well bore an emulsion of an aqueous solution of an alkali metal silicate in a liquid hydrocarbon, thereafter subjecting the solution adjacent the well bore to microwave energy to break the emulsion, releasing alkali metal silicate to form a gel through contact with formation brines and thereby sealing off the remainder of the formation from the well bore. Over a period of time the emulsion in the remainder of the formation will break and release alkali metal silicate to gel and plug the remainder of the formation. Where salinity of the formation brine is insufficient to gel the alkali metal silicate in a reasonable period of time, a gelation agent is incorporated in the hydrocarbon liquid as a second internal phase to accelerate gelling.